The invention relates to a method for shaping hollow boards of thermoplastic material which have at least one gas pocket having a wall, wherein the hollow board to be shaped is heated.
Known from practical application are methods of the aforementioned kind, with which hollow boards of thermoplastic material of the aforementioned configuration are heated, thereby softened, and, subsequently, shaped in the softened state, for example, by means of deepdrawing devices or presses. Problems result in the case of the conventional methods because of the fact that the medium within the gas pocket or gas pockets is heated together with the hollow board and expands as a result of the increase of its temperature. This expansion is correlated with an undesirable deformation of the wall of the gas pockets up to their bursting.
Prior art methods of a different-type are disclosed in EP 0 646 619 A, EP 0 778 310 A, and EP 0 558, 989 A.
In this connection, EP 0 646 619 A concerns a method for manufacturing shaped parts of foamed polymer particles, wherein in this context the employed polymer particles are to be shaped for adaptation to the employed mold. EP 0 778 310 A describes a method for manufacturing polyolefin foamed particles wherein the pre-foamed particles are further foamed and accordingly also deformed. EP 0 558 989 A finally concerns a method for manufacturing structured surfaces on shaped foam bodies which is directed to dissolve the structure of the foam forming the shaped body in the concerned surface area by treatment with heat and pressure.
It is an object of the invention to eliminate the above disadvantage of the prior art of the aforementioned kind.
According to the invention this object is solved in that during the course of a method of the aforementioned kind the wall of the gas pocket or gas pockets is pressure-loaded externally over the full surface area and the hollow board to be shaped is heated while such a full surface area pressure loading of the wall of the gas pocket or gas pockets is carried out. As a result of the full surface area pressure loading, a pressure is exerted externally onto the wall of the gas pocket or gas pockets which counteracts the inner pressure increasing within the gas pocket or gas pockets as a result of heating of the hollow board. Accordingly, the aforementioned wall is force-loaded on its inner side as well as on its outer side and, for a technically proper adjustment of the external pressure to the inner pressure resulting from heating of the gas pocket or gas pockets, the wall is protected from undesirable deformation and damage. In an ideal situation, the inner pressure of the gas pocket or gas pockets as well as the externally applied pressure coincide at any time during the heating and shaping process. It can therefore be expedient to increase the pressure exerted externally onto the wall of the gas pocket or gas pockets with increasing heating of the hollow board.
In a preferred embodiment of the method according to the invention, the wall of the gas pocket or gas pockets is pressure-loaded externally at least over parts of the surface by means of a flowable pressure medium. By employing a flowable pressure medium the full surface area pressure loading of the wall of the gas pocket or gas pockets can be ensured particularly reliably. As a result of its flowability, such a pressure medium is particularly able to penetrate into the interior of the hollow boards to be shaped and to generate the desired external pressure on the walls of the gas pocket or gas pockets present therein. Moreover, when using a flowable pressure medium, contact between the wall of the gas pocket or gas pockets or the surface of the hollow board to be shaped and a pressure piston, which contact may otherwise possibly be required, can be avoided or at least limited. According to the invention, there is the possibility of supporting wall parts by means of abutments which are, for example, formed by contact surfaces on the device for performing the method according to the invention and of loading, at the same time, other wall parts by means of the flowable pressure medium. It is also possible according to the invention to load the full surface area of the wall of the gas pocket or gas pockets externally by means of the flowable pressure medium.
In a further preferred embodiment of the method according to the invention, a flowable pressure medium is used whose temperature surpasses the initial temperature of the hollow board to be shaped. As a result of this measure, it is possible to employ the pressure medium not only for generating the external pressure required for the protection of the gas pocket or gas pockets but, moreover, to utilize it for heating the hollow board.
In the context of the method according to the invention, it is basically possible to employ any flowable pressure medium, in particular, any gaseous pressure medium or any pressure liquid. The flowable pressure medium preferred according to the invention is pressurized steam or heated pressure oil.
An advantageous variant of the method according to the invention for shaping hollow boards with two spaced apart cover layers and at least one gas pocket arranged between the cover layers at a spacing from their edges is characterized in that the edges of the cover layers are compressed in a substantially seal-tight way with formation of at least one cavity adjoining externally the wall of the gas pocket or gas pockets and in that the hollow board is pressure-loaded from the exterior on at least one of the cover layers and is heated during such loading. As a result of the aforementioned measures of the method, a full surface area external pressure loading of the wall of the gas pocket or gas pockets can be achieved by an active pressure loading of only at least one of the cover layers of the hollow board. The cavity or cavities in the interior of the hollow board, which are the result of the afore described pressing of the board edges, contain a medium, preferably air, which corresponds with respect to its thermal expansion behavior to the contents of the gas pocket or gas pockets, in particular, air contained therein. Accordingly, the contents of the cavity or cavities will expand by the same amount as the contents of the gas pocket or gas pockets of the hollow board. The resulting inner pressure of the cavity then provides in the form of an external gas pocket pressure the support of the wall of the gas pocket or gas pockets.
In a preferred embodiment of the method according to the invention, it is suggested that at least one of the cover layers of the hollow board is loaded externally by a flowable pressure medium.